How to create a bootable installer for macOS

You can use an external drive or secondary partition as a startup disk from which to install the Mac operating system.

These advanced steps are primarily for system administrators and others who are familiar with the command line. You don't need a bootable installer to install macOS, but it can be useful when you want to install macOS on multiple computers without downloading the installer each time.

Download macOS from the App Store

Download macOS from the App Store, using a Mac that is compatible with the macOS you're downloading.To download High Sierra, use a Mac with High Sierra, Sierra 10.12.5 or later, or El Capitan 10.11.6. Enterprise administrators, please download from Apple, not a locally hosted software-update server.

When the macOS installer opens, quit it without continuing installation.

Find the installer in your Applications folder as a single ”Install” file, such as Install macOS High Sierra.

Use the 'createinstallmedia' command in Terminal

After downloading the installer, connect the USB flash drive or other volume that will be used as the bootable installer. Make sure that it has at least 12GB of available storage.

Open Terminal, which is in the Utilities folder of your Applications folder.

Type or paste one of the following commands in Terminal. These assume that the installer is still in your Applications folder, and the name of your volume is MyVolume. If your volume has a different name, replace MyVolume accordingly.

A bootable installer doesn't download macOS from the Internet, but it does require the Internet to get information specific to your Mac model, such as firmware updates. If you need to connect to a Wi-Fi network, use the Wi-Fi menu in the menu bar.